A liquid crystal display generally works by applying an electrical field to liquid crystal molecules in a liquid crystal layer placed between a pair of substrates to change a direction of the molecules to which they are aligned and thereby change optical characteristics of the liquid crystal layer.
A liquid crystal display provided with a switching element, e.g., thin-film transistor, for each pixel, has an electrode at each of a pair of substrates which hold a liquid crystal layer in-between, and is designed to apply an electrical field to the layer in a direction essentially perpendicular to the substrate interface. This type of display, known as the so-called actively driven display, is represented by a twisted nematic (TN) display which works on optical rotation of the liquid crystal molecules constituting the liquid crystal layer. One of the largest disadvantages to be solved for this type of display is narrow viewing angle.
On the other hand, an IPS type display has been developed. It works on birefringence produced in the liquid crystal layer by its liquid crystal molecules rotating essentially in parallel to the substrate, where an electrical field generated by an inter-digital electrode provided on one of a pair of the substrates is designed to have a component running essentially in parallel to the substrate surface. This type of display, disclosed by, e.g., Patent Documents 1 and 2, has been offering promising prospects as a novel display which can replace the conventional TN type, because of its advantages in, e.g., wider viewing angle and lower-load capacity, resulting from in-plane switching of the liquid crystal molecules, and hence rapidly advancing recently.
Patent Document 1 discloses a liquid crystal display which has an aligned film placed between spacer beads and a liquid crystal display, and is made of a material capable of exhibiting a liquid crystal alignment capacity when irradiated with polarized light to secure a contrast of up to 320. Patent Document 2 discloses a liquid crystal display which irradiates a high-molecular-weight film with linear polarized light to secure a contrast of up to 250.
The IPS type liquid crystal display is a promising technique for monitors and TV sets of large display area, because of its excellent viewing angle characteristics (e.g., contrast ratio and tone/hue reversal) and bright images it produces. This type of display is hereinafter referred to as “IPS-TFT-LCD.”
A liquid crystal display is provided with a liquid crystal alignment layer having a liquid crystal alignment capacity in the interface between each of a pair of substrates and liquid crystal display placed between these substrates. For an IPS-TFT-LCD to be commercialized for large display areas (20-inch type or larger), however, it is necessary to develop a novel structure and process which allow uniform alignment treatment over the large display area (large-size panel).
Uniform alignment treatment of a liquid crystal alignment layer over a large area is difficult, in particular for an IPS-TFT-LCD, which has a number of stepped structures on a surface facing the liquid crystal layer. This type of display has a significantly narrower margin for alignment treatment of a liquid crystal alignment layer than a conventional TN type, in particular a normally open type displaying bright images at a low voltage and dark images at a high voltage, which is now a prevailing type. The narrower margin comes from the following reasons (1) to (3), described below.
(1) Stepped Structure
An IPS-TFT-LCD should have a number of elongate electrodes several microns wide (sometimes referred to as inter-digital electrode) for its working principle. As a result, it has a number of fine stepped structures.
The step size, determined by thickness of the electrode and shape of a varying film formed on the electrode, is normally 10 nm or more. A high-transmittance pixel structure has a thick inorganic insulation film, and stepped irregularities thinner than the inorganic film are flattened to some extent.
Therefore, the steps on a liquid crystal alignment layer in a high-transmittance pixel structure mainly come from the electrodes on an organic insulation film, where a liquid crystal alignment layer (sometimes referred to as aligned layer) of high-molecular weight compound, e.g., polyimide, serves as the uppermost film.
A conventional mass production technique rubs a liquid crystal alignment layer to impart a liquid crystal alignment capacity (initial alignment) thereto. Rubbing cloth is composed of fine fibers, about 10 to 30 μm thick, tied in bundles. Essentially, each of these fine fibers locally gives a shear force to the aligned layer to realize a liquid crystal alignment capacity.
There are very fine fibers, about several microns thick. These fibers, however, have not been commercially used for rubbing cloth, because the fibers are required to have a rigidity to realize a frictional force to some extent.
An IPS-TFT-LCD has electrodes arranged at intervals of the same order of the fiber thickness, i.e., about 10 to 30 μm, and cannot be sufficiently rubbed in the vicinity or the steps. Therefore, its alignment tends to be distorted. The distorted alignment will deteriorate image quality resulting from increased black level, which in turn results in decreased contrast ratio and uneven brightness.
(2) Alignment Angle
An IPS-TFT-LCD should have an initial alignment direction deviated by a certain angle or more from the direction perpendicular to or in parallel to the electrode extending direction, because of its working principle. The electrode may be an electrode for a signal interconnection, common electrode in a pixel, or pixel electrode.
In order to set an initial alignment direction by rubbing, it is necessary to rub the aligned layer with about 10 to 30 μm thick fibers in a given angle direction, as discussed above. These fibers, however, are dragged from a given angle towards the step in the presence of an interconnection and the step at the end of the interconnection, where the interconnection contains a signal-transmitting electrode, common electrode in a pixel or pixel electrode, extending in a certain direction, to distort the alignment and thereby deteriorate image quality resulting from decreased black level or the like.
(3) Lower Black Level
Efficient lower black level (black display) is one of the characteristics of an IPS-TFT-LCD. As a result, distorted alignment in this type of display is more visible than in others.
A conventional normally open type TN display produces a black level (dark level) at a high voltage. The dark level is determined by the relationship between liquid crystal molecule disposition and polarizer plate position, because most of the molecules are aligned in the electrical field direction, which is perpendicular to the substrate surface, at a high voltage. Therefore, uniformity of the dark level is rather insensitive to the initial alignment condition at a low voltage for its working principle.
The human eye recognizes uneven brightness as relative brightness ratio, which changes essentially on a logarithmic scale. It is therefore sensitive to dark level changes. A conventional normally open type TN display, which forcibly aligns the liquid crystal molecules in one direction at a high voltage, is more advantageous than an IPS-TFT-LCD also in this respect, because it is less sensitive to the initial alignment condition.
On the other hand, an IPS-TFT-LCD produces a dark level at a low or zero voltage, and is more sensitive to the distorted initial alignment condition. In particular, polarized light entering a liquid crystal layer runs with the linear component kept essentially undistorted in the birefringence mode, where liquid crystal molecules are aligned in parallel to each other on the upper and lower substrates (homogeneous alignment), and the light transmission axis in one polarizer plate is set in parallel to the liquid crystal alignment direction and that in the other plate perpendicular to the alignment direction. This design is effective for subduction of the dark level. Transmittance T in the birefringence mode is generally represented by the following formula:T=T0·sin2{2θ(E)}·sin2{(π·deff·Δn)/λ}
Wherein, To is a coefficient mainly determined by transmittance of the polarizer plate for the liquid crystal panel, θ(E) is angle between alignment direction of the liquid crystal molecules (effective optical axis of a liquid crystal layer) and polarized light transmission axis, E is intensity of the electrical field applied, deff is effective thickness of the liquid crystal layer, Δn is anisotropy of refractive index, and λ is light wavelength.
The product of effective liquid crystal layer thickness deff and liquid crystal refractive index anisotropy Δn, i.e., deff·Δn, is defined as retardation. It should be noted that liquid crystal layer thickness deff is not thickness of the whole liquid crystal layer but the liquid crystal layer portion actually changing in alignment direction in an electrical field.
Because, no change in alignment direction occurs in the liquid crystal molecules in the vicinity of the liquid crystal layer interface, even when a voltage is applied thereto, due to the anchoring effect at the interface. Therefore, the relationship deff<dLC invariably holds, wherein dLC is thickness of the whole liquid crystal layer placed between the substrates. The difference between them can be estimated at about 20 to 40 nm, although varying depending on type of the interface in which a liquid crystal panel material and the liquid crystal layer are in contact with each other, e.g., on type of aligned layer material.
Referring to the above formula, sin2{2θ(E)} is the term which depends on electrical field intensity. This means that brightness can be controlled by changing electrical field intensity E in accordance with the angle θ.
A polarizer plate for a normally closed type display is set in such a way to keep the angle θ at 0 when no voltage is applied, which makes the display sensitive to distortion of the initial alignment direction.
As discussed above, uniformity of alignment is an essential parameter for IPS-TFT-LCDs, which has clarified problems involved in the current rubbing method.
The rubbing alignment treatment generally involves various problems, e.g., TFT failure caused by friction-generated static electricity, unsatisfactory display caused by distorted alignment, which results from distorted ends of fibers for rubbing cloth, or dust, and frequently required exchange of rubbing cloth.
The so-called rubbingless alignment, which produces aligned liquid crystal molecules without using rubbing, has been studied to avoid the problems involved in rubbing treatment, and various approaches have been proposed. One of these approaches is photo-alignment, where a high-molecular-weight film is irradiated with polarized ultraviolet ray.
For example, a method disclosed by Non-patent Document 1 is characterized by aligning liquid crystal molecules in one direction with polarized light while dispensing with rubbing treatment adopted in conventional methods.
The photo-alignment has been attracting attention as a novel liquid crystal alignment approach involving no rubbing treatment for its several advantages, e.g., dissolved problems associated with rubbing treatment (e.g., scratches left on the treated film surface, and those caused by static electricity), and simplified process in consideration of commercial production.
High-molecular-weight compounds have been proposed as materials for films with aligned liquid crystal molecules. They have a photo-reactive group introduced into the side chain for necessity of securing photochemical sensitiveness to polarized light.
Polyvinyl cinnamate may be cited as one of the representative high-molecular-weight compounds. It is considered to exhibit anisotropy in the film of the high-molecular-weight compound to align the liquid crystal molecules, when irradiated with light to dimerize the side chain.
Another example proposed so far is a film of high-molecular-weight compound dispersed with a low-molecular-weight dichroic azo dye, where the film is irradiated with polarized light to align the liquid crystal molecules in one direction.
A still another example is a specific polyimide, where the film is irradiated with polarized ultraviolet ray or the like to align the liquid crystal molecules. The liquid crystal alignment is considered to occur, because the main polyimide chains extending in one direction are decomposed when irradiated with light.
Patent Document 1: JP Patent No. 3,303,766
Patent Document 2: JP-A-11-218765
Non-patent Document 1: W. M. Gibbons, et. al., Nature, vol. 351, 49 (1991)